1. Field of Invention
The present invention relates to a method for manufacturing a piston for an automobile engine, particularly a method for manufacturing a piston for an automobile engine which can prevent casting defects such as oxide inclusions or shrinkage cavities on the surface of a combustion chamber over a piston head after casting.
2. Description of Related Art
A piston in a vehicle engine is a part reciprocating within a cylinder to transmit power to a crankshaft through a connecting rod connected to its bottom while being pushed down by the force of a high-temperature and high-pressure gas in the power stroke.
The piston head is a portion of the combustion chamber and the combustion chamber is an element having large influence on performance and fuel efficiency in the engine, such that a structure that allows high-efficiency combustion of an air-fuel mixture with the minimum environmental pollution is required. The shape of the combustion chamber depends on the mounting position of the valve train and the ignition plug, the cooling type, and the shape of the piston head etc.
Recently, as an engine technology, a Gasoline Direct Injection (GDI) type of engine has been introduced to meet the increasing need for the development of a high-pressure injector technology and improve fuel efficiency. The GDI type has the advantage of improving engine performance and fuel efficiency and reducing exhaust gas, because it directly inject fuel into a combustion chamber and then burns it. In particular, the GDI type has the advantage of allowing ultra lean combustion with high combustion safety, as compared with a Port Fuel Injection (PFI) type (a type of supplying air-fuel mixture into a combustion chamber by injecting fuel into an intake port), which is an indirect injection type.
It is important in the GDI engines to collect well fuel, which is directly injected into the combustion chamber, around the ignition plug and this depends on the structure of the combustion chamber, accordingly the shape of the piston head that is a portion of the combustion chamber is also important.
A concave bowl that can control the flow of air and fuel is usually formed on the top of the piston (the head top) in the GDI engines in order to resolve the problem in mixing engine oil into the mixture, improve engine performance and fuel efficiency, and reduce Particular Matters (PMs) and exhaust gas. The bowl allows production of a dense mixture in the ignition area around the ignition plug by guiding the flow of the air and fuel.
That is, while the air introduced to the bowl through the intake port flows on the bowl wall, the flow is enhanced, and the fuel injected from an injector mixes with the air while vaporizing after hitting against the bowl wall, such that a stratified mixture is produced, and then combustion is made with the mixture rarefied throughout the combustion chamber, but stratified and dense around the ignition plug.
As described above, by forming the bowl to burn a dense mixture in the ignition area, it is possible to improve engine performance and fuel efficiency and to reduce particular matters and exhaust gas. Further, the shape of the top of the piston head with a bowl, which is a portion of the combustion chamber, influences engine oil dilution, engine performance and fuel efficiency, and production of particular matters and exhaust gas.
Accordingly, various shapes of bowls for maximizing combustion performance and fuel efficiency and minimizing exhaust gas have been proposed, and Korean Patent Application Publication Nos. 10-2004-0041308 and 10-2009-0064171 and Korean Patent No. 10-0946484 can be exemplified as prior art documents for a piston with a bowl. Further, as a method of manufacturing a piston head, gravity die casting is widely used, and Korean Patent Application Publication No. 10-2002-0024678 and Korean Patent No. 10-1009962 can be exemplified as prior art documents for a manufacturing method using gravity die casting.
In general, the gravity die casting means a casting method of making a casting by injecting molten metal into a mold, using gravity, and is widely used for manufacturing pistons, sleeves, crankcases, cylinders, and bearings etc., using non-ferrous metal alloys such as aluminum (Al), magnesium (Mg), and copper (Cu), cast iron, and steel.
In the process of manufacturing a piston, using the gravity die casting, molten metal is poured into a mold by a ladle and molded/hardened for a predetermined time, and then the molded product is cooled by opening the mold. After the product is cooled, the riser formed in casting is removed by turning or milling and a bowl is formed, and then following machining for a skirt, a ring groove, a pin hole, and an oil hole is performed, thereby completing a piston head.
Recently, as the shape of the top of a piston head, including a bowl and a combustion chamber, is complicated for the combustion characteristic, it becomes difficult to remove a riser and form a bowl. In particular, casting defects such as oxide inclusions and shrinkage cavities and a problem in durability is generated with the complication of the shape and additional machining for removing the defected portions is required.
FIGS. 1 and 2 are views illustrating problems in the related art. FIG. 1 shows the flow path of molten metal poured inside through a gate of a mold in manufacturing a piston, and FIG. 2 is a view illustrating that an oxidized film is formed in a combustion chamber by a change in flow of molten metal due to the concave shape of a bowl on head top.
In a piston that is used for a GDI engine, a concave bowl (12 in FIG. 2) is supposed to be formed, as described above, on the top 11 of the piston head, and as shown in FIG. 1, a riser 13 is positioned inside the bowl in casting.
As molten metal is poured inside through a gate 2, the molten metal rises up from the bottom in a mold 1 and, in the normal case, an oxidized film that is formed on the surface of the molten metal is supposed to be moved and collected at a riser 13 on the top of the piston and then the oxidized film is supposed to be removed with the riser 13 without remaining on the piston head.
However, since the shape of the top 11 of the piston head is complicated by a bowl, the flow of the molten metal changes in direction at the curved portion such as the bowl (the flow of the molten metal is changed by the concave shape of the bowl) and a large amount of oxidized film is produced on the top of the head (on the surface of the combustion chamber).
As shown in FIG. 2, the oxidized film on the surface is blocked and not moved to the riser 13 by the curved portion and remains as an oxide inclusion on the surface of the bowl 12 on the top 11 of the head (the surface of the combustion chamber). Further, the molten metal does not smoothly flows on the surface of the bowl 12, such that many casting defects such as shrinkage cavities are generated.
FIG. 3 is a picture showing that oxide inclusions remain on the concave curved portion of a bowl on a piston manufactured by a manufacturing method of the related art and FIG. 4 is a picture showing shrinkage cavities and oxide inclusions on the surface of a combustion chamber after gravity die casting.
Since the shapes of bowls on pistons are recently complicated to improve a combustion characteristic, the possibility of a casting defect generated on the surface of the head top (the surface of a combustion chamber) is further increased. Since the casting defects are the basic reasons for damage of pistons, it is required to preclude casting defects on the surface of the combustion chamber or remove casting defects through additional machining after casting, in order to improve durability of the pistons.
The additional machining causes a problem that limits the shape of the bowl, for example, a circular bowl for turning (machining the inside of the bowl), in addition to reduction of productivity, and requires milling for other shapes than a circle. In particular, the more complicated the shape is, the more difficult the machining is and even 3D NC machining is required to remove casting defects by machining the complicated surface of the top of a piston head, and therefore, productivity may be considerably decreased (milling for the top of a bowl and 3D NC machining for the outside of a bowl).
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.